Granulocytes and monocytes, collectively called myeloid cells, are differentiated descendants from common progenitors derived from hematopoietic stem cells in the bone marrow. Commitment to either lineage of myeloid cells is controlled by distinct transcription factors followed by terminal differentiation in response to specific colony-stimulating factors and release into the circulation. Upon pathogen invasion, myeloid cells are rapidly recruited into local tissues via various chemokine receptors, where they are activated for phagocytosis as well as secretion of inflammatory cytokines, thereby playing major roles in innate immunity. Genetic alterations in myeloid cells may cause an abnormal increase in mature myeloid or blast cells resulting in chronic or acute myelogenous leukemia.